The conversion of incident solar light to thermal energy has recently become of widespread interest. Since Kirchoff's Law joins together absorptivity and emissivity, most attempts to improve solar absorption have involved the development of materials which have high absorptivity in the solar wavelengths (visible spectrum, mainly) and low emissivity in the system operating temperatures (near infrared black body radiation for an operating temperature of 550.degree. C which is typical of steam pressure used in turboelectric generators).
The devices fabricated using this concept are multilayered structures, called interference stacks or bulk-absorber stacks. See "Physics Looks at Solar Energy" by A. B. Meinel et al, appearing in Physics Today, February 1972, pp. 44-50. These stacks create a selective surface that is black for wavelengths shorter than 1.3 microns and mirrorlike for longer wavelengths. Thus the stacks serve to create a surface having a double function, namely, high absorptivity over the solar emission band and low emissivity over the blackbody emission range and thus lend themselves to use as efficient converters of thermal energy into heat reservoirs. These devices have problems of stability at moderate temperatures, such as 550.degree. C, and demand submicron thickness tolerances over the wide areas necessary for solar conversion. So little is known about thin film interaction and diffusion, that film stability has been the major obstacle in the operation of these devices.
The present invention converts photon energy to heat by the use of an absorbing surface which is a geometric maze whose microstructure is similar in geometry to an acoustic anechoic surface. The optical photon absorber surface consists of a dense forest of aligned needles of dimensions of the order of visible wavelengths with a spacing between such needles of the order of several wavelengths of visible light. Such a surface is believed to absorb with a high efficiency because of multiple reflections occurring as the incident photons penetrate the needle maze in a manner similar to that in which absorption takes place in an anechoic chamber because of multiple reflections of sound. For a narrow incident cone surrounding the direction of the needles, the maze has an absorptivity approaching 1. However, only a small part of the hemispherical emissivity is concentrated in this narrow cone. Thus, by making the needles of the solar energy converting device of a low emissivity metal, e.g., tungsten, the total integrated hemispherical emissivity of the device is considerably less than 1.
Consequently it is a primary object of this invention to make a device in such a manner that the material will be highly absorbing within a narrow cone of incident light but have a very low hemispherical emissivity over the black body radiant wavelengths at the operating temperature of the device.